The following is an extract from our nature newsletter Wild Wild Life. Sign up to receive it for free in your inbox every month.
I’ve completely non-dramatically been referring to this spring and summer as slugmageddon. If you’re a gardener in northern Europe, you probably know what I mean – the mild winter and wet spring has led to staggering numbers of slugs and snails. Every morning I dread looking out the window to discover the night’s damage, and you, like me, may now have a burgeoning collection of sad, denuded stems and empty pots.
I’ve noticed, though, that my culinary herbs are doing relatively well. The mint has been nibbled and I do find slugs on the thyme, but they – along with the rosemary, sage, oregano, chives and lavender – are all, so far, hanging on. So this month I’m looking at the science of smelly, tasty plants and their effects on humans and other organisms.
Advertisement
As the lavender begins to bloom in the front gardens where I live, I’ve been thinking about its renowned calming scent. Lavender fragrance is used in many commercial products, particularly ones that are meant to be relaxing, so it’s tempting to think that the way lavender makes us feel is a learned association. But last month żěè¶ĚĘÓƵ reported on an experiment that found that spraying lavender scent into pig pens leads to these animals being less aggressive and appearing more relaxed, suggesting there is a biochemical basis to lavender’s relaxing effects.
Lavender, like many plants with a strong scent and taste, gets its aroma from multiple chemicals, and in lavender’s case, linalool is a key part of the mix. This alcohol molecule has been found to reduce anxiety in mice, and it’s thought that linalool can influence our central nervous systems through a number of mechanisms. But that’s likely to be a happy accident – the smelly compounds produced by lavender probably evolved to repel insects and protect against microbes. Linalool has been found to change the activity of a gene involved in nerve signalling in beetles, for example.
Deploying defensive chemicals that we enjoy smelling and eating is a common theme among herbs. When I asked my colleagues for their favourites, a difficult consensus ultimately formed around rosemary, basil and mint, all of which contain flavoursome molecules involved in plant defence. The cineole (also known as eucalyptol) in rosemary, for example, has been found to disrupt various functions in disease-causing bacteria, fungi and viruses, and the eugenol in basil is antimicrobial too. Mint is an interesting one, as the menthol it contains stimulates receptors our bodies use to detect cold, making mint taste cool and fresh. It’s pleasant for us, but less so for flies – a 2019 study found that when menthol stimulates the same receptors in fruit fly larvae, they roll up to try to protect themselves from pain or injury.
Another żěè¶ĚĘÓƵ staff favourite is coriander (or cilantro), which is a controversial pick because a large minority of people find it distastefully “soapy”. That’s due to differences in our DNA, most commonly affecting the gene for an odour receptor that detects aldehyde molecules. Coriander is an extreme example, but we all have different tastes when it comes to herbs – some people can’t abide the flavours of aniseed, liquorice and fennel (which all contain anethole), and several of my colleagues professed a hatred of dill, which contains both anethole and a lot of carvone, a chemical also found in caraway and spearmint. It’s intriguing to think that much of this might be genetic. A 2020 study of more than 2000 people in Iceland found a number of gene variants that affected their ability to detect the smells of anethole, cinnamaldehyde (from cinnamon) and fish odour – something to bear in mind next time you’re frustrated by the seemingly wilful dietary aversions of a dinner party guest.
While herbs seem to have initially evolved their aromatic molecules to defend themselves from predation and disease, for thousands of years they’ve been shaped by people selectively breeding cultivars that enhance our favourite flavours. Thai basil has a more aniseed-like taste than the Genovese kind that is preferred for pesto making, and there’s now a huge variety of mints that evoke the taste of everything from pineapple to chocolate. We have also cultivated herbs for medicinal uses for thousands of years, making the most of their diverse chemical toolkit.
But I wonder if, as well as us shaping their evolution, herbs might also have shaped ours? The way lavender makes us feel calm or mint makes us feel cool might originally have been of no benefit to us or the plants, which were simply defending themselves from predators and pathogens. But if using these herbs both as medicines and as food additives helped to protect us from disease, pests and microbes, then perhaps evolution might have encouraged us to develop more of a taste for them. If the vast fields of farmed lavender I see on Instagram this time of year are anything to go by, it’s a relationship that has paid off for the plants.
